Interplay of atomic displacements in the quantum magnet (CuCl)LaNb2O7

Abstract

We report on the crystal structure of the quantum magnet (CuCl)LaNb2O7 that was controversially described with respect to its structural organization and magnetic behavior. Using high-resolution synchrotron powder x-ray diffraction, electron diffraction, transmission electron microscopy, and band structure calculations, we solve the room-temperature structure of this compound [alpha-(CuCl)LaNb2O7] and find two high-temperature polymorphs. The gamma-(CuCl)LaNb2O7 phase, stable above 640K, is tetragonal with a(sub) = 3.889 A, c(sub) = 11.738 A, and the space group P4/mmm. In the gamma-(CuCl)LaNb2O7 structure, the Cu and Cl atoms are randomly displaced from the special positions along the {100} directions. The beta-phase [a(sub) x 2a(sub) x c(sub), space group Pbmm] and the alpha-phase [2a(sub) x 2a(sub) x c(sub), space group Pbam] are stable between 640 K and 500 K and below 500 K, respectively. The structural changes at 500 K and 640 K are identified as order-disorder phase transitions. The displacement of the Cl atoms is frozen upon the gamma --> beta transformation, while a cooperative tilting of the NbO6 octahedra in the alpha-phase further eliminates the disorder of the Cu atoms. The low-temperature alpha-(CuCl)LaNb2O7 structure thus combines the two types of the atomic displacements that interfere due to the bonding between the Cu atoms and the apical oxygens of the NbO6 octahedra. The precise structural information resolves the controversy between the previous computation-based models and provides the long-sought input for understanding the magnetic properties of (CuCl)LaNb2O7.

12 pages, 10 figures, 5 tables; crystallographic information (cif files) included

References in corpus (6)

• Modified kagome physics in the natural spin-1/2 kagome lattice systems - kapellasite Cu3Zn(OH)6Cl2 and haydeeite Cu3Mg(OH)6Cl2
• A Novel Ordered Phase in SrCu2(BO3)2 under High Pressure
• Structural distortion and frustrated magnetic interactions in the layered copper oxychloride [CuCl]LaNb(2)O(7)
• Electronic structure and magnetic properties of Li_2ZrCuO_4 - a spin 1/2 Heisenberg system in vicinity to a quantum critical point
• Atomic and magnetic structures of (CuCl)LaNb${}_2$O${}_7$ and (CuBr)LaNb${}_2$O${}_7$: Density functional calculations
• Synthesis, Structural and magnetic properties of the solid solution (CuCl1-xBrx)LaNb2O7 (0<x<1)

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• Microscopic model of (CuCl)LaNb2O7: coupled spin dimers replace a frustrated square lattice
• Ground-state phase diagram of the spin-1/2 square-lattice J1-J2 model with plaquette structure
• (CuCl)LaTa2O7 and quantum phase transition in the (CuX)LaM2O7 family (X = Cl, Br; M = Nb, Ta)
• Short-range order of Br and three-dimensional magnetism in (CuBr)LaNb2O7